1. Field of the Invention
The present invention relates to a novel polymerizable, radiopaque dental composition, especially for fillings for teeth.
2. Description of the Prior Art
A majority of the commercial tooth-filling compositions have only a slight X-ray absorption. As a result, it is not possible for the treating dentist to see, for example, an existing tooth filling, or to be able to differentiate the filling from the surrounding tooth material, on an X-ray photograph made with equipment used in the dentist's office. An examination of the existing filling is thus only superficially possible, especially after the filling has been in place for a longer period of time. The formation of marginal gaps or cracks along the edges, changes in the surrounding tooth material (especially below the filling), as well as loss of material cannot be controlled by the dentist with the non-radiopaque dental compositions; thus, such compositions could lead to further damage to the teeth of the patient.
Furthermore, excess filling material, which can result in difficult-to-see locations (for example in the approximal region), is often difficult to discover if it doesn't show up on an X-ray photograph.
This result, especially in recent times, has increasingly led to the sale of a series of polymerizable, radiopaque dental compositions. Commercial preparations generally contain barium, strontium, lanthanum, or zinc-containing glass or silica, such as known Muller of April 3, 1975, for example from German Offenlegungsschrift No. 23 47 591, and U.S. Pat. Nos. 3,808,170 Rogers dated Apr. 30, 1974 and 3,975,203--Dietz dated Aug. 17, 1976, or radiopaque additives together with other commercial fillers, such as quartz, certain lithium aluminum silicates, silicic acids, silica gel, or silicic acid granules. Examples of radiopaque additives include barium sulfate, zirconium oxide, or lanthanum oxide. U.S. Pat. Nos. 3,971,754--Jurecic dated July 27, 1974 and 3,801,344--Dietz dated Apr. 2, 1974 disclose ceramic filler compounds which, among other things, contain oxides of lanthanum, hafnium, and the lanthanide series metals. For an optimum application, radiopaque dental compositions should have a greater X-ray visbility than does human dentine. Normally, the X-ray visibility of materials is expressed in mm aluminum per mm material. For example, human dentine has an X-ray visibility of approximately 1.5 mm aluminum. In other words, radiopaque dental compositions should have an X-ray visibility of greater than 1.5 mm aluminum.
However, with the aforementioned, heretofore known additives, and the ceramic filler compositions, such an X-ray visibility is achieved only if at the same time a loss in the transparency of the polymerized material is accepted. Unfortunately, this means that the cosmetic appearance of this material can no longer be optimally adapted to the surrounding tooth material. The transparency of the polymerized materials depends largely on the ratio of the index of refraction of the filling material to the polymer matrix. Although the indexes of refraction of monomers and polymers do not differ much, being in the range of 1.45 to 1.6 for commercially available polymerizable dental compositions, radiopaque additives do in fact exhibit great differences; in particular, the heretofore known radiopaque additives have an index of refraction of greater than 1.6. However, dental compositions having too great an opacity not only have a cosmetic drawback but, where preparations are involved that should be hardened with light, additionally lead to an insufficient depth of polymerization, since the light required for the curing can no longer penetrate deep enough into the composition. This often leads to faulty applications, with non-polymerized material existing below the cured surface; the consequence of this non-polymerized material can then be further damage to the tooth. In addition, the quality of the cured or hardened composition is unsatisfactory due to the non-cured material disposed below it.
For this reason, most commercial radiopaque dental compositions contain no radiopaque additives. Rather, all or at least a very great proportion of the filler content comprises radiopaque glass or silica, with which a sufficient transparency and satisfactory X-ray visibility can be achieved. Unfortunately, tooth-filling compositions having such glass fillers can no longer achieve the physical properties that can be achieved with other fillers. These last-mentioned compositions are generally hydrolytically susceptible, which means they can be washed out at the surface over a period of time. Their color stability is sometimes not satisfactory, and above all their physical properties deteriorate far quicker than do those of compositions filled with other fillers, such as quartz. Thus, due to the lesser hardness of glass relative to quartz, tooth-filling compositions produced with glass are less resistant to abrasion. Furthermore, it is not possible to grind the glass as fine, so that tooth-filling compositions that can be highly polished cannot be produced with such glass fillers. Up to now this was only possible with so-called micro-filler preparations, or with tooth-filling compositions that contain granules of such very small primary particles. A dental composition that is capable of being highly polished should contain only such filling material that the primary particle sizes of less than 1 .mu.m. Glass ground this fine becomes opaque due to the grinding process that is necessary to accomplish this, so that again the same drawbacks result as with other radiopaque additives.
An object of the present invention therefore is to provide novel, polymerizable, radiopaque tooth-filling compositions that do not have the drawbacks of the known compositions and with which, especially simultaneously, an excellent X-ray visibility and excellent optical properties are realized.
This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying Examples.